630
HENRYRAPOPORT AND CLYDE D. WILLSON
urnished 18,19-dihydro-l9-~orynantheonea7 (31 mg. ), m .p. 223-25'. Further elution with ether-methanol (9: 1) gave ajmaliciol(32 mg.). The ketone so obtained was exposed to a Wolff-Kishner reduction as described p r e v i o ~ s l yexcept l ~ ~ that the reaction time was reduced from 4 t o 3.25 hours resulting in a consistent 6 2 7 yield ~ of dihydrocorynantheane. N.-Methylyohimbane.*-To yohimbane (60 mg.) in dry benzene (4 ml.) potassium (10 mg.) was added and the mixture was stirred under reflux in a nitrogen atmosphere for several hours, until the metal was consumed. The suspension was cooled to room temperature and treated with an excess of methyl iodide in benzene. The reaction mixture was stirred overnight and the precipitate (120 mg.) of potassium iodide and Na-methylyohimbane methiodide was collected and dried. When this mixture (40 mg.) was heated at 280-300" in DUCUO, it gave a sublimate which afforded Namethylylohimbane, m.p. 178-179' (lit. 179') from methanol. The total yield after processing all the salt mixture was 38 mg. N,-Methylylohimbane with excess methyl iodide in benzene gave the methiodide, m.p. 287-288' (lit. 288-289'). Repetition of the above vacuum pyrolysis a t 280-300° regenerated the tertiary base. N.-Methy1dihydrocorynantheane.-Following the procedure described above, dihydrocorynantheane (10 mg.) in dry benzene (4ml.) was treated with potassium (6 mg.). Two additions of methyl iodide (0.5 ml.) a t 45-minute intervals to the resulting potassium salt gave after 15 hours a white precipitate (20 mg.). Pyrolysis of this in vucuo a t 340' gave a crystalline sublimate (6 mg.) which afforded N.-methyldihydrocorynantheane, m.p. 109-110.5', [CY]D -22', from aqueous methanol. Anal. Calcd. for CzoHzeN~:C, 81.03; H , 9.52; N, 9.45. Found: C, 79.71; H,9.52; N,9.24. N,-Methylcorynantheidane .-Corynantheidane (60 mg.) was converted by the procedure above into its potassium derivative in dry benzene and alkylated with excess methyl iodide. A portion (50 mg.) of the crude salt mixture (127 mg.) was vacuum pyrolyzed a t 280-320' to furnish an oily sublimate. This was dissolved in 57&acetic acid and treated with a few drops of perchloric acid to give n'.-methylcorynantheidane perchlorate (25 mg.), m.p. 208-210' after recrystallization from methanol. Anal. Calcd. for C Z O H Z B N Z . C ~ O ~ . C H C,~ O 57.78; H : H, 7.89. Found: C, 57.54; H, 7.48. NS-Methyl-3,4,5,6-tetradehydroyohimbanePerchlorate. (a).-N.-Methylyohimbane (20 mg.) was catalytically dehydrogenated in an aqueous solution on a steam-bath by palladium black (10 mg.) using maleic acid (40 mg.) as an acceptor. The hot reaction mixture was filtered and gave (37) This ketone was refluxed in methanolic sodium methoxide and recovered unchanged. As a consequence, its previously assumed stereochemistry (&id iii) has been corroborated. (38) Similar t o a procedure described by B. Witkop [ J . A m . Chsm. Soc., 76, 3361 (1953)j for the same compound.
[CONTRIBUTION FROM
THE
VOl. 84
upon addition of perchloric acid Na-methyl-3,4,5,6-tetradehydroyohimbane perchlorate, m.p. 250-251 (b).-3,4,5,6-Tetradehydroyohimbane perchlorate (20 mg.) was dissolved in a small volume of methanol and treated with a few drops of 10% sodium hydroxide. The deep yellow solution was diluted with water until a precipitate began to form and then was extracted with ether. The extract was dried, evaporated and the residue in dry benzene was treated with 2 drops of methyl ptoluenesulfonate. The resultant precipitate (9 mg.) had m.p. 205'. It was dissolved in a minimum of methanol and treated with perchloric acid to furnish Na-methy1-3,4,5,6-tetradehydroyohimbane perchlorate, m.p. 250-251' after crystallization from methanol. The perchlorate could also be obtained by passing a solution of the p-toluenesulfonate through a column of Amberlite resin [CG45 (Clod)]. Anal. Calcd. for CmH2rN,.C10,: C, 59.64; H, 6.43; N, 6.62. Found: C, 59.37; H, 6.24; N, 6.58. d-trans-2,3-Diethyl-112,3 ,4-tetrahydro-12-methylindolo[2,3-a] quinolizinium Perchlorate (trans-X), (a) .-N.Methyldihydrocorynantheane (5 mg.), maleic acid (10 mg.) and palladium black (4 mg.) were heated in water on a steambath with stirring for 13 hours. Filtration and then addition of perchloric acid to the cooled solution gave the perchlorate (4 mg.), m.p. 198-200', [ a ]+ 1~5 O , after crystallization from aqueous methanol. (b).-Dihydrocorynantheane (90 mg.) was heated in a sealed evacuated tube with palladium black (90 mg.), maleic acid (200 mg.) and water (4ml.) for 36 hours, when the ultraviolet absorption spectrum indicated that conversion to the tetradehydro compound was complete. After filtration and basifying with 25% sodium hydroxide, the anhydro compound was extracted into methylene chloride which was dried and evaporated. The residue was heated with methyl bromide for 10 minutes in a sealed tube a t loo', then dissolved in water and a drop of perchloric acid was added. The perchlorate recrystallized from aqueous ethanol had m.p. 200". [a]D $5' (4MeOH-lCHCla). Anal. Calcd. for CzoHzaN2.Cl0,: C, 61.14; H , 6.41. Found: C, 61.4; H, 6.5. Both synthetic samples had infrared spectra identical with the degradation product of isoajmaline and the mixed m.p.'s showed no depression. l-~is-2,3-DiethyI-l,2,3,4-tetrahydro-12-methylindolo [2,3a]quinolizinium Perchlorate (X).-A portion (30 mg.) of the salt mixture obtained above from the methylation of corynantheidane was vacuum pyrolyzed a t 300-340". The oily sublimate was dissolved in alcohol and added to an aqueous solution of maleic acid (60 mg.) and suspended palladium black (20 mg.). The whole was heated on a steam-bath for 16 hours. The solution was filtered hot and the filtrate plus hot water washings were concentrated to remove most of the ethanol. The cooled solution was treated with perchloric acid to yield the @-carboliniumperchlorate (13 rng.). Recrystallization from aqueous methanol gave the pure salt, m.p, 212-214', [CY]D -27" (CHCIs). The infrared spectrum was identical with the degradation product derived from ajmaline and the mixed m.p. showed no depression.
'.
DEPARTMENT OF CHEMISTRY, UNIVERSITY OF CALIFORNIA, BERKELEY 4, CALIF.]
The Preparation and Properties of Some Methoxypyrroles BY H E N R Y RAPOPORT AND CLYDED. WILLSON] RECEIVED MAY11, 1961 Using ethyl glycinate-l-14C derivatives, a series of cyclizations by which these compounds yield 3-pyrrolidones has been shown to involve the carbonyl and not the methylene group of the glycine residue. Dimethyl ketals or methoxypyrrolines derived from these pyrrolidones give methoxypyrrolecarboxylic acids on catalytic dehydrogenation. From these acids and others, several unsymmetrical dipyrryl ketones also have been prepared. A comparison of the spectral properties of these compounds with those of the prodigiosin precursor showed that the prodigiosin precursor could not be a methoxydipyrryl ketone.
At one stage prior to the synthesis of prodigiosin,2 thesis of the then postulated tripyrrylmethene we had considered various approaches to the syn- structure I, for prodigiosin. The key substance (1) Public Health Service Predoctoral Research Fellow of the National Institute of hlental Ilcalth, 1058-1960.
(2) H . Rapoport and K 0 Holden, J (1962).
A m . Chem SOL., 84, 635
SOMEMETHOXYPYRROLES
Feb. 20, 1962
in our consideration was the compound C10HlON202, which had been clearly established as a prodigiosin precursor3 and differed from prodigiosin by the elements of methylamylpyrrole. This precursor of necessity would have a pyrryl methoxypyrryl ketone structure, 11, if the tripyrrylmethene structure were correct. The published properties of this precursor did not appear inconsistent with such a structure, although no methoxypyrryl ketones were known at that time. For these reasons we undertook and here report the preparation of some methoxypyrroles and related ketones.
631
in sodium ethoxide-ethanol solution yielded only the pyrrolidone IVb. Thus t h i s method, although applicable for the ultimate synthesis of 4-alkoxypyrrole-2-carboxylic acids (derivable from IVa) and 4-alkoxypyrrole-3-carboxylicacids (derivable from IVb, c), was not adaptable for the synthesis of 3-alkoxypyrrole-2-carboxylic acids, for which the unrealized isomer V would be needed. COOCZH;
CHZCOOCZHZ
I
__f
NHCOOCZHS
CH,,
+
CHR
I
,CHR
/
COOC2H5
111;: C,
a, RI = OCHI, Rc = H b, RI = H, Rc = OCfi
Lu
CHZCOOC2H5
I
CHCOOC2Hs 11
I
I COOCZHS
133gOOC2H5 R=CHB
I
jA
Va, R=COOC2HS
b, R = H Two methods are available for the preparation C, R=CH3 of unsymmetrical dipynyl ketones, both involving I c the reaction of a pyrrolecarboxylic acid chloride with a second pyrrole, either under Friedel-Crafts I COOCzHS conditions4~b or with the pyrryl Grignard ' V R Thus the corresponding methoxypyrrolecarboxylic IVa, R = COOC2H5 acids were needed, and a promising approach apb, R = H dooc2H5 C, R=CH3 peared to be present in suitable modifications of VIa, R = COOCzH5 the method of Kuhn and Osswalds who had reb, R = H ported the synthesis of 4-ethoxypyrrole-2-carC, R=CH3 boxylic acid. Conversion of the ethyl l-ethoxycarbonyl-4Their procedure involved as the first step condensation of ethyl N-ethoxycarbonylglycinate and oxopyrrolidine-2-carboxylate (VIa) to the cordiethyl fumarate to give the reported diethyl 1- responding pyrrole paralleled the known procedures ethoxycarbonyl-4-oxopyrrolidine - 2,3 - dicarboxylate through the dimethyl ketal VII, which could be (IVa) directly. This condensation is capable of thermally converted to a mixture of the various yielding either of two isomers (IVa and Va), or a possible methyl enol ethers VIII. The final aromamixture of both. Since their subsequent objective tization to the 4-methoxypyrrole-2-carboxylicacid was the decarboxylated ketone VIa, i t was not (IXa) , using the recommended N-bromosuccinnecessary to prove the structure of the inter- imide, however, proceeded in much poorer yield mediate pyrrolidone. However, in order to extend than was reporteds for the homologous 4-ethoxy this method to the synthesis of other alkoxypyrrole compound. Direct catalytic dehydrogenation of acids, i t was essential to know the direction of ring the mixed enol ethers VIII in various solvents also gave low yields of a mixture of methoxy methyl closure in this type of reaction. We therefore utilized ethyl N-ethoxycarbonyl- ester I X b and the demethoxylated methyl ester X , gly~inate-l-'~Cin these syntheses with three isolated by hydrolysis and re-esterification with different a,@-unsaturated esters, going directly diazomethane. However, catalytic dehydrogenation of the ketal to the ring-closed product and decarbethoxylating the resulting (3-ketoesters. The carbon dioxide VI1 resulted in the direct formation of ethyl 4evolved, and the pyrrolidones resulting from these methoxypyrrole-2-carboxylate (IXc) , probably decarbethoxylations were then analyzed for radio- arising by cracking of the 1-ethoxycarbonyl group activity. I n every case the carbon dioxide was (see below). Hydrolysis and esterification with inactive and essentially all of the activity remained diazomethane gave pure methyl 4-methoxypyrrolein the pyrrolidones. Therefore, with each com- 2-carboxylate (IXb) in reasonable yield. The same type of pyrrolidone intermediate was pound, the direction of ring closure was the same and exclusively that shown by arrow A, rather utilized for the synthesis of 4-methoxypyrrole-3than the alternative direction B. Similarly, ring carboxylic acid (XIIb). For this purpose, the pclosure of the isolated 14C-labeled triester I I I b ketoester IVb was treated with diazomethane to form the methyl enol ether XI, and this was de(3) U. V. Santer and H. J. Vogel, Federalion Proc., 15, 1131 (1956): hydrogenated directly to ethyl C-methoxypyrroleBiochem. Biophys. Acta, 19, 578 (1958). (4) H. Fischer and H. Orth, Ann., S02, 237 (1933). 3-carboxylate (XIIa) by heating a t 235O with ( 5 ) R. Huisgen and E. Laschtuvka, Ber., 98, 65 (1960). palladium-on-carbon. During the dehydrogena(6) V. V. Chelintsev and D. K. Skvortzov,J . Russ. Phys. Chcm. SOC., tion ethylene and carbon monoxide were evolved, 47, 170 (1915). undoubtedly arising by cracking of the l-ethoxy(7) B. Oddo, Gars. chim. ita!., S O , 267 (1920). ( 8 ) R. Kuhn and G . Osswald, Bcr., 89, 1423 (1956) carbonyl group.
HENRYRAPOPORT AND CLYDE D. WILLSON
632
clear that the methoxypyrroles would not withstand such vigorous treatment. A feasible alternative was found in the use of oxalyl ~ h l o r i d e . ~ Formation of acid chloride may be followed conveniently by carbon dioxide evolution as the sodium salt of the acid is warmed with oxalyl chloride. Without isolation of the acid chloride, the solution is then treated with the pyrryl Grignard reagent to form the resultant dipyrryl ketone. When applied to the sodium salt of pyrrole-2carboxylic acid (XIII), 100 mole per cent. of carbon dioxide was evolved and a 95% yield of 2,2'dipyrryl ketone (XIV) was obtained. However,
VIa
/ 2
I
COOCZHS
COOCzHS
VI1
i!COOR N
H IXa, R = H b, R=CHS C, R=CzHs CIIdOu(:OO(l~H~J,~H;O 1Vh LH \ I'd/( (
H X
- u"""': +
-
S I
Vol. 84
OW
.H
XI
co
H
+
HA('=( )Ii
XIIa It=(',H 1, I i = H
Since the direction of pyrrolidone formation precluded the preparation of any other isomeric methoxypyrrole acids by this procedure, the further compounds desired for comparison were prepared by the reaction of ethyl N-ethoxycarbonylglycinate with diethyl ethoxymethylenemalonate. These were a 2-methoxy pyrrole-3-carboxylic ester, and 3-methoxy- and 5-methoxypyrrole-2carboxylic esters. The position of longest wave length absorption for the various methoxypyrrole esters is presented in Table I along with the corresponding values for the methyl analog. It can be seen that in practically every instance the effect of the methyl and methoxyl substituent is quite similar. In those cases (eight of the thirteen compounds) where the acid was also measured, it was found to show a hypsochromic shift of 2-5 mp.
XIV
with pyrrole-3-carboxylic acid and 4-methoxypyrrole-2-carboxylic acid only about 50 mole per cent. of carbon dioxide was evolved and the yields of 2,3'-dipyrryl ketone (XV) and 4-methoxy-2,2'dipyrryl ketone (IIb) were correspondingly lower. We have attempted to explain these differences, and particularly the decreased carbon dioxide evolution, by postulating an intramolecular cyclization to the easily substituted a-position in the case of the 0-acid
R
TABLEI ULTRAVIOLET ABSORPTION* OF SOMEPYRROLE CARBOXYLICSuch a cyclization might also occur to the Pposition in the more nucleophilic methoxypyrrole. OF THE EFFECTOF METHYLAND ESTERS: COMPARISON We have summarized the physical properties of METHOXYL SUBSTITUENTS Xmsi,
Amam
Compound
mp
Compound
mp
the dipyrryl ketones and prodigiosin precursor in Table 11. Obviously, the methoxydipyrryl ketone TABLE I1 PROPERTIES OF SOMEDIPYRRYL KETONES
3-CH3 268b 2-CHa 255e 3-OCHa 264" 2-0CH8, 1-C& 257' 4-CH3 272d 4-CHa 227d 4-OCH3 286 4-OCH3 234 5-CH3 2 785 5-CHs 262* 5-OCH8, 1-CH3 286c a In methanol. * M. Scrocco and R. A. Nicolaus, Atti accad. naz. Lincei, Rend., Classe sci. fis., mat. e. nut., 22, 311 (1957). Ref. 3. R . Oesterlin, unpublished observations, this Laboratory. e Ref. 13.
We next turned our efforts to the synthesis of some methoxypyrryl pyrryl ketones. For this purpose, the corresponding acid chlorides were needed, and in the past these have been prepared from the acids by the action of thionyl chloride or phosphorus pentachloride. I t was immediately
absorption, Carbonyl Ketone
XIV
XV
IIb
M.P., OC.
Amax,
m r a (e)
mr*
160-161
257 (5,350) 334 (25,000)
6.26
99-100
249 (7,500) 312 (16,680)
6.22
tI
130-131
260 (6,140) 6.27 347 (20,730) Prodigiosin precursorc >250 254 (13,000) Amide ( ? ) dec. 363 (35,000) Inmethanol. * In chloroform. Ref. 3. (I
(9) A. L. Wilds and C . H. Shunk, J . A m . Chein Soc.. 70, 2427 (10481
Feb. 20, 1962
SOMEMETHOXYPYRROLES
633
1-ethoxycarbonyl -4,4 -dimethoxypyrrolidine -2 -carboxylate (VII); infrared absorption, Xmax 5.74(s), 5.90(s) p . Anal. Calcd. for Ci2H210aN:C, 52.3; H , 7.7; N, 5.1; OR, 4.00/275. Found: C, 52.3; H, 7.6; N, 5.1; OR, 3.95/ 275. Methyl 4-Methoxypyrrole-2-carboxylate (IXb).-In a dehydrogenation vessel equipped with a carbon dioxide sweep inlet, reflux condenser and magnetic stirrer were placed 20.0 g. (0.073 mole) of ethyl l-ethoxycarbonyl-4,4dimethoxypyrrolidine-2-carboxylate (VII), 15 g. of 5% palladium-on-carbon and 50 ml. of diisopropylbenzene. This mixture was vigorously stirred and boiled (220') for 8 hours, after which gas evolution had virtually ceased. The reaction mixture was filtered, the catalyst was washed with chloroform several times, and the combined chloroform washes and filtrates were then fractionally distilled. The mixture, boiling from 120-135' (1.5 mm.), crystallized in the receiver and was recrystallized from benzene to yield ethyl 4-methoxypyrrole-2-carboxylate (IXc), m.p. 55-58' ; ultraviolet absorption, , A, 238 mp ( e 8100), 288 (10,100). Anal. Calcd. for CaHllOaN: C, 56.8; H , 6.6; N, 8.3; OR, 2.00/169. Found: C, 56.6: H , 6.6; N, 8.2; OR, 1.95/ 169: In a parallel experiment only the diisopropylbenzene was distilled off from the dehydrogenation reaction mixture, the crude residue then being dissolved in a solution of 30 g. of potassium hydroxide in 250 ml. of methanol which was boiled for 1 hour. The methanol then was removed under reduced pressure and the residue was diluted with 100 ml. of water and boiled for an additional 15 minutes. The aqueous solution was then washed with two 100-ml. portions of chloroform, acidified a t 5-10' to @H 2.5 with phosphoric acid, and extracted with five 100-ml. portions of ether. A large excess of ethereal diazomethane was added to the combined ether extracts; after standing for 5 hours, the Diethyl l-Ethoxycarbonyl-4-oxopyrrolidine-2,3-dicarbox- yellow-orange solution was washed with 50 ml. of satd. aqueylate (IVa).-The procedure of Kuhn and Osswalds was ous bicarbonate, dried, and evaporated to a residue which modified as follows: To a suspension of 8.6 g. (0.38 mole) of was placed on a column of 120 g. of neutral alumina (acfreshly prepared sodium sand in 1 1. of benzene was added tivity I). Benzene and benzene-chloroform (4: 1)eluted 2.? slowly, with stirring, 66 g. (0.38 mole) of ethyl X-ethoxy- g. of a yellow crystalline material which was sublimed a t 40 carbonyl-glycinate and, after 6 hours, 65.0 g. (0.38 mole) of (0.5 mm.) and recrystallized from methanol to yield 2.3 g. diethyl fumarate, dissolved in about 100 ml. of benzene, was (20 yo) of methyl 4-methoxypyrrole-2-carboxylate (LXb), added. The addition was completed in 15 minutes, and m.p. 85-86', infrared absorption, Xmsr 2.92(m), 5.84(m), the suspension was allowed to stir a t room temperature for 5.90(s), 6.31(s), 7.47(s), 9.01(s), 10.03(m), 10.39(m)fi; 3 hours and then boiled for 30 min. ultraviolet absorption, XmaI 237 mp ( E 8080), 286 (10,020). The reaction mixture then was cooled to 0', 200 ml. of Anal. Calcd. for C7Hs03N: C, 54.2; H, 5.9; N, 9.0; ether was added followed by 600 ml. of ice-water, and the organic phase then was removed and washed with two 200- CHsO, 40.0. Found: C, 54.1;H, 5.9; N, 9.0; CHaO, 39.9. ml. portions of ice-water. The combined aqueous solutions Methyl Enol Ethers (VIII) of Ethyl l-Ethoxycarbonyl-4were washed with 250 mi. of ether and poured over a mix- oxopyrrolidine-2-carboxylate (Via).-Ethyl l-ethoxycarture of 12 ml. of concd. sulfuric acid and 150 g. of cracked bonyl-4-oxopyrrolidine-2-carboxylate (VIa)* (74 g., 0.32 ice. This mixture was extracted five times with 200-1111. mole), 38.5 g. (0.35 mole) of dimethyl sulfite,ii 50 ml. of portions of chloroform, and the combined chloroform ex- abs. methanol and 3 ml. of a satd. solution of hydrochloric tracts were washed with 100 ml. of aqueous bicarbonate, acid in methanol were boiled for 6 hours, then the contents dried, and distilled to yield 70 g. (61%) of diethyl l-ethoxy- of the flask were distilled until drops of methanol ceased to carbonyl-4-oxopyrrolidine-2,3-dicarboxylate, b.p. 175-177" distil (bath temp. 200' for 1 hour). The residue was dis(1.0 mm.) (reporteds b.p. 175-177" (1.0 mm.)). The solved in 400 ml. of ether, the ether solution was washed semicarbazone was crystallized from ethanol; m.p. 188-189" with 100 ml. of 1 N sodium hydroxide and 50 ml. of water, (reported8 m.p. 183-185'). then dried and distilled a t 137-142" (1.0 mm.), yielding 71 Ethyl l-Ethoxycarbonyl-4,4-dimethoxypyrrolidine-2-car- g. (93%) of a slightly yellow liquid, characterized as a mixboxvlate iVII).-Ethvl 1-ethoxvcarbonvl-4-oxonvrrolidineture of the three possible methyl enol ethers VIII; infrared 2-c&boxylate 1VIa)a (37 g., 0.18mole), 19.2 g. (01175 mole) absorption: Xmax 5.73(s), 5.87(s), 6.04(m)p; ultraviolet of dimethyl sulfite," 100 ml. of abs. methanol and 2 drops absorption: Xmax 223 mp ( E 3500), which did not shift in 1 of a satd. solution of methanolic hydrochloric acid were N HC1 or in KOH-methanol. Gas phase chromatography heated under reflux for 6 hours, then cooled and distributed on a silicone column a t 183" separated this material into between 1500 ml. of chloroform and 500 ml. of satd. aqueous three peaks. bicarbonate. The aqueous portion was washed with 150 ml. Anal. Calcd. for C1lHITOSN: C, 54.3; H , 7.0; N, 5.8; of chloroform, and the chloroform extracts were combined, OR, 3.00/243. Found: C, 54.1; H , 7.2; N, 5.7; OR, dried and fractionally distilled. After a small fore-run 2.95/243. appeared a t 110-120" (0.8 mm.), the main product was col4-Methoxypyrrole-2-carboxylic Acid ( E a ).-To a solution lected a t 124-125' (0.8 nim.), yielding 41 g. (93y0)of ethyl of 5.6 g. of potassium hydroxide in 25 ml. of 50% aqueous methanol was added 3.1 g (0.02 mole) of methyl 4-methoxy(10) All melting points were taken on a Kofler block and are corrected; boiling points are uncorrected. Infrared spectra were taken in pyrrole-2-carboxylate (IXb), and the mixture was boiled for 1 hour. It was then distributed between 250 ml. of water chloroform and ultraviolet spectra were taken in methanol, Microanalyses and pK, determinations were performed by V. Tashinian, and 100 ml. of ether. The aqueous phase was separated Microchemical Laboratory, University of California, Berkeley. and acidified with phosphoric acid a t 5-10' to PH 2.5, and Radioactivity was determined by wet combustion as barium carbonate then extracted with five 100-ml. portions of ether. The and specific activities are reported as disintegrations/minute/millimole combined ether extracts were dried and evaporated t o a (d.p.m./mmole). VnIess otherwise specified, a nitrogen atmosphere volume of about 50 ml. and then diluted slowly with 100 was maintained during all reactions. ml. of chloroform. On cooling, a crystalline precipitate ( 1 1 ) C . M. Suter and H. L. Gerhart, "Organic Syntheses," Coil. formed which was sublimed a t 100" (0.1 nim.), yielding 2.45 Vol. TI, John Wiley and Sons, h-ew York. 1943, p. 112. g. (80%) of 4-methoxypyrrole-2-carboxylic acid (IXn), m.p.
IIb is quite different from prodigiosin precursor, and by interpreting the data of Table I1 in conjunction with those of Table I i t is possible to predict that the isomeric ketone, 3-methoxy-2,2'-dipyrryl ketone (IIa) will also be quite different from prodigiosin precursor. Thus, the effect of the 4-methoxy substituent on the ultraviolet absorption of methyl pyrrole2-carboxylate (Table I) is a bathochromic shift in the long wave length maximum of 22 mp. Correspondingly, the effect of a 4-methoxy group on the spectrum of 2,2'-dipyrryl ketone is a bathochromic shift of 13 mp. In the isomeric series, there was no effect on the position of maximum absorption of methyl pyrrole-2-carboxylate when substituted with a 3-methoxy group, both compounds absorbing a t 264 mp. Correspondingly, the long wave length maximum of 3-methoxy-2,2dipyrryl ketone would be expected to differ only slightly if a t all from that of 2,2'-dipyrryl ketone itself, which differs considerably from the reported3 values for prodigiosin precursor (Table 11). From these considerations, we concluded that prodigiosin precursor could not be either methoxydipyrryl ketone 11, and that the tripyrrylmethene structure (I) for prodigiosin was untenable. Experimentallo
634
HENRY RAPOPORT AND
CLYDE
D.
WILLSON
VOI. 84
179-BO', pK. 4.2; infrared absorption:, , X 2.97(w), of benzene. Evolution of carbon dioxide began immediately 6.00(s), 632(s) p ; ultraviolet absorption: , A, 235 mp and 97 mole per cent. was evolved in 45 minutes. At this ( e 7970), 281 (8300), and in 0.1 N methanolic potassium point, 10 mmoles of pyrryl Grignard reagent (freshly prehydroxide, 232 (7830), 270 (7970). pared from pyrrole and ethylmagnesium iodide) in ether was Anal. Calcd. for CsH70aN: C, 51.1: H , 5.0; N, 9.9; added, the mixture was stirred for an additional 30 min. a t OCHs, 22.0; equiv. wt., 141. Found: C, 51.3; H, 5.2; N, 50°, and 30 ml. of a satd. aqueous ammonium chloride solu10.0; OCH8, 21.9; equiv. wt., 143. tion was added. The aqueous phase was separated and exEthyl l-ethoxycarbonyl-4-oxopyrrolidie-3-carboxylate tracted with five 20-ml. portions of chloroform, these chloro( I n ) was prepared by exactly the Same procedure as form extracts were combined with the original benzene exabove for the 2,3-dicarboxylate IVa, using an equivalent tract, the combined extracts were washed with 30 ml. of amount of ethyl acrylate in place of diethyl fumarate. The aqueous bicarbonate solution, dried and evaporated to a product was distilled a t 125-130' (0.8 mm.) and melted at residue, which was recrystallized from methanol and sub60-62" (reported8 m.p. 59-62'); infrared absorption: limed a t 120" (0.5 mm.), yielding 1.2 g. (75y0)of 2,2'-dipyrryl ketone (XIV), m.p. 160-161O (reported12 m.p. 160Xmax 2.96(w), 5.69(s), 5.84-6.00(s), 6.12(s) p , ultraviolet 161'); infrared absorption: Anlax 2.94(s), 6.26(s), 6.35(s), absorption: Xmax 246 m p ( e 6200), in 1 N methanolic potas6.45(s) p; ultraviolet absorption: Xma, 257 mfi ( e 5360), sium hydroxide, 275 (14,300). Ethyl l-Ethoxycarbonyl-4-methoxy-A~-pyrroline-3-car-291sh (6630), 334 (25,000). 2,3'-Dipyrryl ketone (XV) was prepared in the same way boxylate (XI).-A large excess of ethereal diazomethane was as the 2,2'-isomer described above, starting with pyrrole-3added to a solution of 69.0 g. (0.3 mole) of ethyl l-ethoxycarbonyl4oxopyrrolidine-3-carboxylate (IVb) in 500 ml. of carboxylic acid.18 Carbon dioxide evolution reached only 35 ether. After standing for 3 hours the reaction mixture was mole per cent. The product was purified by chromatography evaporated under reduced pressure, and the residue was dis- on alumina (neutral, activity I ) , eluting with benzene-chlorosolved in 500 ml. of ether, washed with 200 ml. of 1 Nsodium form, and crystallization from chloroform, from which it hydroxide, dried, and fractionally distilled. Ethyl l-eth- crystallized as the solvate. Sublimation a t 50" (0.1 rnm.) oxycarbonyl-4-methoxy-A8-pyrroline-3-carboxylate ( X I ) yielded 2,3'-dipyrryl ketone (XV), m.p. 99-100'; infrared was obtained in 60 g. (830/0) yield at 160-162' (2.0 mm.) and absorption: XmX 2.97(s), 6.22(s), 6.30(s); ultraviolet absorption: Xmax 249mp ( e 7500), 312 (16,680). rapidly crystallized, m.p. 65-66'; infrared absorption: Anal. Calcd. for GH80N2: C, 67.5; H, 5.0; N, 17.5. Amax 5.85-5.95(s), 6.08(s)p; ultraviolet absorption: Xmax Found: C, 67.4; H. 4.9; N, 17.5. 248 mp ( e 7400). Anal. Calcd. for C11HlTOsN: C, 54.3; H , 7.0; N, 5.8; 4-Methoxy-2,2'-dipyrryl ketone (IIb) was prepared in the OR, 3.00/243. Found: C, 54.0; H, 7.2; N, 5.9; OR, 3.02/ same way as the two dipyrryl ketones above, starting with 243. 4-methoxypyrrole-2-carboxylic acid ( IXa). Carbon dioxide Ethyl 4-Methoxypyrrole-3-carboxylate (XIIa).-In a de- evolution reached 55 mole per cent. Purification on aluhydrogenation vessel fitted with a stirrer, reflux condenser mina (neutral, activity I), eluting with chloroform, suband carbon dioxide sweep were placed 5.0 g. (0.02 mole) of limation a t 100' (0.02 mm.), and crystallization from abs. ethyl l-ethoxycarbonyl-4-methoxy-As-pyrroline3-carbox-ethanol gave 4-methoxy-2,2'-dipyrryl ketone ( IIb), m.p. ylate (XI), 3.0 g. of 5% palladium-on-carbon and 40 ml. of 130-131O; infrared absorption: XmX 2.96(m), 6.27(s), di-n-hexyl ether. The system was heated a t 235' for 3 6.36(s)p; ultraviolet absorption: ,A, 260mp ( e 6140), 347 hours after which gas evolution had practically ceased. (30,730). The reaction mixture was filtered, the catalyst was washed Anal. Calcd. for Ci0Hl0O2N2:C, 63.1; H , 5.3; N, 14.7; several times with chloroform, and the combined filtrate OCHs, 16.3. Found: C, 62.7; H, 5.2; N, 14.8; OCHJ, and washings were dried and distilled from a short-path still. 16.1. The product was a viscous liquid, boiling a t 130-145' (0.7 Experiments with Glycine-1-14C. Ethyl N-ethoxycarmm.), which crystallized and was recrystallized from ben- bonylglycinate-1-14C was prepared by standard procedures14 zene and sublimed a t 103" (0.1 mm.), yielding 700 mg. (21%) of ethyl 4-methoxypyrrole-3-carboxylate(XIIa), m.p. 107- from g l y ~ i n e - l - ~ ~ C . Condensations of ethyl N-ethoxycarbonylglycinate-l-14C 109"; infrared absorption: Xmax 2.91(m), 3.06(w), 5.88with ethyl fumarate, ethyl acrylate and ethyl crotonate to 5.95(s), 6.36(s) p ; ultraviolet absorption: Amax 233 mp ( e yield the radioactive pyrrolidones IVa, N b and IVc, re12,500), 265sh (2185). spectively, were carried out in benzene and in abs. ethanol. Anal. Calcd. for.CBHIIOIN: C, 56.8; H , 6.6; N, 8.3; A. Condensations in benzene have been described above OR, 2.00/169. Found: C, 57.3; H , 6.4; N, 8.5; OR,2.01/ with ethyl fumarate and ethyl acrylate for the preparation of 169. IVa and IVb. In exactly the same manner, ethyl crotonate The gas collected over potassium hydroxide in the eudi- gave ethyl 1-ethoxycarbonyl-2-methyl-4-oxopyrrolidhe-3ometer was transferred to an infrared gas cell and its infrared carboxylate (IVc), b.p. 121-123' (1.0 mm.) (reporteds b.p. absorption spectrum examined; it was found to contain 105-106" (0.02 mm.)). ethylene and carbon monoxide. B. Condensations in absolute ethanol were carried out 4-Methoxypyrrole-3-carboxylic Acid (XIIb).-The ester by adding ethyl X-ethoxy~arbonylglycinate-l-'~Cto an XIIa was hydrolyzed exactly as described above for the 2- equimolar solution of sodium ethoxide in ethanol. A solucarboxylate IXb. Methoxypyrrole-3-carboxylic acid ( IXb) tion of a molar equivalent of the corresponding a,@-utlsatuwas crystallized from methanol-benzene; m.p. 203-204' rated ester in ethanol then was added and the solution was dec., pK. 5.6; ultraviolet absorption: Amax 231 mp ( e boiled for 2 hours. Most of the ethanol was removed i n 10,876), 265sh (2060), in 0.1 N methanolic potassium hy- OQCUO, the residue was distributed between benzene and icedroxide, 220sh (8230). water, the benzene phase was washed with two additional Anal. Calcd. for C&f@aN: C, 51.1; H, 5.0; equiv. wt., portions of water, and the combined aqueous phase acidified 141. Found: C, 51.0; H, 5.0; equiv. wt., 141. to pH 1.0. Extraction with chloroform and fractional disMethyl 4-Methoxypyrrole-3-carboxylate(XIIc).-Esteritillation of the chloroform extracts yielded the pyrrolidones fication of the acid X I I b with diazomethane and crystalliza- IVa, IVb and IVc in substantially the same yield as when the tion of the product from ether-hexane, yielded methyl 4- condensation was performed in benzene. methoxypyrrole-3-carboxylate (XIIc), m.p. 115-117' after Alternative Preparation of Ethyl l-Ethoxycarbonyl-4sublimation a t 60" (0.1 mm.); infrared absorption: Xmax oxo-pyrrolidine-3-carboxylate( I n ) . - T o a solution of 50 6. 2.94(ni), 5.88-5.94(s), 6.35(s) p ; ultraviolet absorption: (0.36 mole) of ethyl glycinate-l-14C hydrochloride in 100 ml. Amax 234 m p ( e 12,070), 265sh (2240). of ice-water was added 36 ml. of cold 1 ,V sodium hydroxide Anal. Calcd. for C7HgOsN: C, 54.2; H , 5.9. Found: solution and then 60 g. of ethyl P-bromopropionate, 210 ml. C, 54.4; H , 6.0. of methanol and a solution of 24 g. of potassiurn carbonate in 2,2'-Dipyrryl Ketone (XIV).-A solution of 1.11 g. (10 275 ml. of water. After being stirred a t room temperature mmoles) of pyrrole-2-carboxylic acid in a minimum quantity for 24 hours, the solution was concentrated in oacuo to 400 of water was titrated to pH 8.0 with 0.1 N sodium hydroxide nil., diluted with 100 ml. of water, and extracted with three 100-ml. portions of chloroform. Distillation of the chloroand the solution was evaporated to dryness under reduced pressure and dried overnight a t 100" (0.01 mm.). The an(12) G. Ciamician and P Magnaghi, Bcr.. 18, 414 (1886). hydrous powder was suspended in 25 ml. of benzene and to (13) H.Rapoport and C D U'illson, J . Org. Cham , 2 6 , 1102 (1861). this suspension immersed in an oil-bath a t 50" was added (14) E. Fischer and E. Otto, B e y . , 36, 2106 (1903). 1.22 g. (9.5 millimoles) of oxalyl chloride dissolved in 25 ml.
SYNTHESIS OF PRODIGIOSIN
Feb. 20, 1962
form extracts yielded 29 g. (39%) of ethyl N-(2-ethoxycarb~nylethyl)-glycinate-l-~~C, b.p. 107-108° (2.5 mm.). Anal. Calcd. for CpHl,04N: C, 53.2; H, 8.4; N, 6.9. Found: C, 53.1; H, 8.4; N,6.9. The ethyl N-(2-ethoxycarbonylethyl)-glycinate-l-14C( 16 g., 0.08 mole) was added to a solution of 8 g. of sodium carbonate in 100 ml. of water and to this was added 8.4 g. of ethyl chloroformate. The reaction mixture was stirred a t room temperature for 4 hours, then adjusted to PH 2.0 and extracted with five 50-ml. portions of benzene. Distillation of the dried benzene extracts yielded 20 g. ( 8 5 % ) of ethyl N-ethoxycarbonyl-N-( 2-ethoxycarbonylethyl)-glycinate-l-14C (IIIb), b.p. 146147' (3.5mm.). Anal. Calcd. for CluHplOeN: C, 52.4; H, 7.7; N, 5.1; OC2H6, 49.1. Pound: C, 52.4; H, 7.9; N, 4.9; OC2H6, 49.4.
[CONTRIBUTION FROM
THE
635
Cyclization of the triester IIIb to the pyrrolidone IVb was carried out in 75% yield in the manner described above for the condensations in absolute ethanol. Decarboxylation of radioactive IVa, IVb and IVc to the pyrrolidones ma, VIb and VIc was carried out as reported.* The carbon dioxide was collected by using a nitrogen sweep and the gas stream was bubbled through molar potassium sulfatepotassium bisulfate ( 2 :1) and then into standard sodium hydroxide solution. Barium chloride then was added to precipitate the barium carbonate for counting. Radioactivity Measurements.-In every case, the starting esters IVa, IVb and IVc had specific activities of 30,000 & 500 d.p.m./mmole. The decarboxylated pyrrolidones VIa, VIb and VIc had the same specific activities as the starting esters, and the evolved carbon dioxide had specific activities of 100 f 30 d.p.m./mmole.
DEPARTMENT OF CHEMISTRY, UNIVERSITY OF CALIFORNIA, BERKELEY 4, CALIF.]
The Synthesis of Prodigiosin1 BY HENRYRAPOPORT AND KENNETH G. HOLD EN^ RECEIVED MAY11, 1961 Prodigiosin, the red pigment of S.marcescens, has been synthesized. The pyrryldipyrrylmethene structure thus established for this pigment was built up through the following stages: (1) condensation of ethyi N-ethoxycarbonylglycinate with diethyl ethoxymethylenemalonate followed by treatment with diazomethane and selective hydrolysis gave ethyl 3-methoxypyrrole-2-carboxylate; (2) heating this ester with A'-pyrroline led to the pyrrolidinylpyrrole which was dehydrogenated t o ethyl 4-methoxy-2,2'-bipyrrole-5-carboxylate;(3) this bipyrrole ester was converted to the corresponding 4-methoxy-2,2'bipyrrole-5-carboxaldehyde; and (4) acid-catalyzed condensation of this aldehyde with 2-methyl-3-amylpyrrole resulted in synthetic prodigiosin, identical with the natural pigment. This is the first synthesis of a pyrryldipyrrylmethene, and prodigiosin (and a related pigment) is the only example of the occurrence of such a skeleton in nature.
Prodigiosin is the red pigment of Serratia marcescens, a widely distributed, non-pathogenic bacterium often found in soil and water. This bacterium, previously known as Bacillus podigiosus, provided the excuse for frequent religious excesses during the Middle Ages when red colonies of the bacillus on consecrated wafers were mistaken for flecks of blood. Prodigiosin itself has considerable antibiotic and antifungal activityJ4but high toxicity precludes its use as a therapeutic agent. The first degradative work reported6 on prodigiosin, CzoH26N30, indicated the presence of three pyrrole nuclei (pyrrole, 3-methoxypyrrole and 2methyl-3-amylpyrrole) joined in some manner by means of the remaining carbon atom required by the empirical formula. On the basis of his work Wrede, in 1933,6bproposed structures I, I11 and IV for prodigiosin, favoring IV in his later publicationsksd without providing any further experimental justification. Nevertheless, because of Wrede's assignment of the tripyrrylmethene structure (IV), other plausible structures were ignored and attention was focused on the synthesis of tri(1) Presented in part as a communicdtion; H. Rapoport and K. G. Holden, J. A m . Chcm. SOC.,8 2 , 5510 (1960). (2) Public Health Service Predoctoral Research Fellow of the National Heart Institute. (3) F. Mayer and A. H. Cook, "The Chemistry of Natural Coloring Matters," Reinhold Publishing Corp., New York, N . Y., 1943,p. 269. (4) P. E. Thompson, D. A. McCarthy, A. Bayles, J. W. Reinertson and A. R. Cook, Anfibiofics and Chcmolhcrapy, 6 , 337 (1956): 0.M. Efimenko, G. A. Kusnetsova and P. A. Yakimov, Biokhim.,21, 416 (1956); 0 . Felsenfeld, D. W. Soman, S. J. Ishibara, T. Waters and J. Norsen, Proc. SOC.E x p f l . B i d . Med.. 77, 287 (1851); for action against coccidioidomycosis, see A. Lack, ibid., 72, 656 (1949); R. E. Weir, R. 0. Egeberg, A. Lack and G. M. Lciby, A m . J . M c d . Sci., 224, 70 (1952). (5) (a) F. Wrede and A. Rotbhass, 2. physiol. Chcm., 216, 67 (1933); (b) 219, 267 (1933); (c) 222, 203 (1933); (d) 226, 95 (1934).
pyrrylmethenes.6-8 However, comparisons of these synthetic model compounds with prodigiosin were inconclusive since the synthetic tripyrrylmethenes differed considerably from prodigiosin in extent and kind of substitution. In fact these comparisons have been interpreted both as evidence fors and againsV.8 the tripyrrylrnethene structure. Synthesis of various other model compounds'"~" led to the proposal'a of a pyridine-containing nucleus (V) for prodigiosin. It was not until very recently12r13that a pyrryldipyrrylmethene structure was again considered for prodigiosin.
y N-
\" H
111, R i = H , R,=CH,O IV, R, = CH30, R,= H
V (6) H. Fischer and K. Gangl, ;bid., 267, 201 (1941). (7) A. Treibs and K. Hintermeier, Ann., 606, 35 (1957). (8)A. J. Castro, A. H. Corwin, J. F. Deck and P. E. Wei. J . Org. Chcm., 24, 1437 (1859). (9) R. Hubbard and C.Rimington, Biochem. J . , 46, 220 (1950). (10) A. Treibs and R. Galler, Angew. Chem., 70, 57 (1958). (11) A. Treibs and R . Zimmer-Galler, Z . physiol. Chem., 318, 12 (1960). (12) G. N a r d and R. A. Nicolaus, Rend. accod. sci. 3s. c mal. (Soc. nad. sci. N a p o l i ) , 26, 3 (1959). (13) H.H.Wasserman, J. E. McKeon, L. Smith and P. Forgione, J . A m . Chcm. SOC.,8 2 , 506 (1960).
